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KE spectra is useful for understanding and tuning the dissipation mechanisms in the model. For a global model, it can be computed via a vector spherical harmonic transform of the instantaneous velocity field (or two spherical harmonic transforms of the instantaneous vorticity and divergence scalars) on a spherical surface. For all the data below, we interpolate to 250mb (I think because this is the most energetic part of the atmosphere - check this?). Spherical harmonics can be thought of as polynomials in Cartesian coordinates (x,y,z) restricted to the sphere. For each degree k there will be 2k+1 polynomials (spherical harmonics) of total degree k. To compute the KE spectra, E(k), we sum the coefficient squared over all degree k spherical harmonics.
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